Saturday, March 5, 2011

Kanban Size Estimation

Is there a formula for Kanban size calculation?
Answer is : “Yes”. But the formula I ever learnt, I feel most of them are very academic and not really practical to the real production environment.

In my previous experience in box build assembly line design, I always consider a Kanban size = 3 into my production system. You may ask why 3, not other lucky number such as 8.

The secret behind is very simple, because my boss say : Kanban size = 3
He demand for 3 is based on his many experience in this field, 3 is the number where he think reasonable and achievable.

Anyway, you can’t just copy the number blindly and apply it to your production system. I’m sure it will not work. You have to go back to your production line, understand how the people run the line then only you can tailor a Kanban size the suite to your production system.

Hahha… talk so much, sound like all nonsense isn’t it.

Ok, lets discuss the case below.

Any Kanban size determination require basic process info as below
  • Takt Time and Raw Process Time (RPT) of your downstream process
  • RPT and Setup Time of your upstream process

There are two WIP Level that need to take care in a Kanban
  • WIP Level in Normal production run
  • WIP Level for “Pull” signal triggering

WIP Level for “Pull” signal triggering is the minimum WIP that enough to keep the downstream process running while waiting for 1st Lot replenishment from the Upstream process. WIP Level above this would be the WIP Level in Normal production run, where we can decide the level based on several considerations, such as

Optimum production time of Upstream process for every conversion
Production storage capacity
Resource availability to handle too many conversion frequencies

For this case, I would determine the “Pull” triggering point as below

Lot # = [(Upstream RPT + Upstream Setup Time) / Downstream RPT] + 1
So, answer is ~3 + 1 = 4

This mean, when the WIP level drop to 4 Lot, a signal will send to Upstream process for Kanban replenishment. But due to long set up time, Upstream process require 130min set up + 40 min production time to deliver the 1Lot to the Kanban tray. Per the RPT given, Downstream process require minimum 3Lot of material in order to keep the line running. I add additional lot in this calculation, because with the consideration Upstream process is not necessary handle one product, as conversion take place. Therefore, additional waiting time may be needed for last Lot clearance before machine conversion work can be start.

For the next level of WIP that supporting normal production run, you need to consider the most economy way to run the process on each conversion. There are company actually set the policy to limit conversion frequency ( such as 2 per day, 4x per day ) with the consideration of conversion cost, productivity and resource availability.

Just my Six Sense

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